This invention relates to a process using a novel crystallization accelerator to greatly increase the crystallization rate of polyesters.
As is well known in the art, polyethylene terephthalate is widely used, e.g., as a fiber, a film, an nonwoven fabric, or the like because of its excellent physical properties. Since, however, its crystallization rate is low, insufficient crystallization generally occurs when using a mold at low temperatures, e.g., from about 80.degree. C. to about 120.degree. C., which temperature range is usually employed in injection molding of thermoplastic polymers, and it has therefore been difficult to produce molded articles of polyethylene terephthalate having the desired physical properties.
In order to increase the crystallization rate of polyethylene terephthalate, a number of crystallization accelerators or crystal nucleating agents have been proposed, including sodium benzoate as disclosed in Japanese Patent Publication No. 29977/71, lithium terephthalate and sodium stearate as disclosed in Japanese Patent Publication No. 14502/72, alkali metal salts of phosphonic acid and phosphinic acid as disclosed in Japanese Patent Publication No. 27140/72.
Many of these crystallization accelerators are salts of weak acids, such as carboxylic acid, or salts of strong acids, such as phosphoric acid and sulfonic acid. The salts of strong acids, however, cannot be dispersed uniformly in polyethylene terephthalate, but are present therein in an insular form as a foreign substance, although their crystallization acceleration effect is great. This leads to unevenness in quality of the resulting polymer, and, furthermore, only a limited portion of the crystallization accelerator added participates in the crystallization acceleration action. With regard to the salts of weak acids, their crystallization acceleration effect is poor, although they can uniformly be dispersed in polyethylene terephthalate. It is therefore necessary to add such a crystallization accelerator in a large amount and, as a result, uniform dispersion of the crystallization accelerator in the polymer cannot readily be attained.